Sanjay Madhavan /J. Pharm. Sci. & Res. Vol. 6(12), 2014, 392-395

Portfolio of Ameloblasts

Sanjay Madhavan Student, Saveetha Dental College Chennai

Abstract-Ameloblasts are cells present only during tooth development that deposit tooth enamel, which is the hard outermost layer of the tooth forming the surface of the crown. In the developing tooth the functioning ameloblast is a tall, narrow cell, with its base attached to the cells of the stratum intermedium. The nucleus is located basally and basal cytoplasm contains abundant mitochondria. The supranuclear cytoplasm contains a large, active Golgi complex and abundant rough endoplasmic reticulum, together with microtubules, which are predominantly longitudinally arranged, and secretory vacuoles which become larger and more numerous near the upper pole. At the upper pole the cell elongates into a single large Tomes’ process, and forms a fringe of smaller processes around its neck. The Tomes’ process contains numerous microtubules and large numbers of secretory vacuoles. The rough endoplasmic reticulum synthesizes various proteins and glycoproteins (including amelogenin and enamelin), which form the organic matrix of enamel (pre-enamel) and are packaged by the Golgi complex into secretory vacuoles. These then move into the Tomes’ process and the small neck processes, discharging their contents on to the surface. Mineralization of the matrix proteins by hydroxyapatite occurs almost instantaneously, producing small enamel crystallites and, with progressive mineralization, the enamel rods or prisms.

Ameloblasts are derived from oral epithelium tissue The outer dental epithelium consisted of short flattened of ectodermal origin. Their differentiation from cells arranged as a multiple layer of cells on the labial preameloblasts is a result of signalling from surface of the apical loop. The cells contained a small, oval the ectomesenchymal cells of the dental papilla. The nucleus which was oriented perpendicular to the surface of ameloblasts will only become fully functional after the first the odontogenic organ. Outside the odontogenic organ were layer of dentin has been formed by odontoblasts. The cells cells and extracellular connective tissue elements. Near the are part of the reduced enamel epithelium after enamel outer dental epithelium several layers of fibroblast-like maturation and then are subsequently lost during tooth cells formed an investing sheath similar to the dental eruption[1,2] sac.The undifferentiated mesenchymal cells of the pulp facing the ameloblasts in this region were heterogeneous in ZONES OF AMELOGENESIS appearance. (1) Presecretory Zone (a) Region of Ameloblasts facing pulp,(ii) anterior portion The entire layer of ameloblasts from the point of greatest (odontogenic organ). convexity of the apical loop until a discrete layer of enamel There was an increased regularity of the junction between was present, was classified into various regions ameloblasts and provisional stratum intermedium. The cells prior to the zone of enamel secretion. of the provisional stratum intermedium were a mixture of (a) Region of Ameloblasts facing pulp,(i) posterior portion squamous and cuboidal shapes, but the latter, containing (odontogenic organ). large irregularly shaped nuclei, predominated. Outside the The apex of the ameloblast faced the the undifferentiated outer dental epithelium, small capillaries came into closer cells of the pulp and the base was adjacent to equally association with the odontogenic organ and disrupted the undifferentiated cells of the provisional stratum investing appearance previously shown by the cells of the intermedium. The stellate reticulum and outer dental dental sac. In the pulp, three changes were noted. epithelium were also undifferentiated. The ameloblasts at First, the cells adjacent to the ameloblasts became this stage were organized into an apparently stratified low progressively organized into cuboidal, then low columnar columnar epithelium with two or three staggered levels of and finally, in the region of ameloblasts facing dentin, into nuclei. The nuclei were large, oval and elongated in the a single layer of differentiated columnar odontoblasts. long axis of the cell. The junctions with the pulp at its apex Second, cells lying deeper in the pulp showed evidence of and the provisional stratum intermedium at its base were organization into two or three layers of flattened cells wavy and irregular. A basement membrane separated the forming a subodontoblastic layer Lastly, large, thin walled ameloblasts from the pulp. The term "provisional stratum capillaries came into close association with the developing intermedium" was applied to the layer of cells lying against subodontoblastic layer . the base of the ameloblasts. These cells were "provisional" because, first, they were continuous with the well- (b) Region of ameloblasts facing dentin organized layer of stratum intermedium seen further It was in this region that the ameloblasts developed all the incisally, and second, because it was often unclear as to morphological features of secretory ameloblasts except for whether a cell in this layer belonged to the ameloblasts, the Tomes, processes. At the apical limit of the region the stellate reticulum or to stratum intermedium itself. appearance of the ameloblasts was more regular than in the previous region, and by the end of the apical one-third of

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this region, mitotic figures were no longer seen in ameloblasts. (3) Maturation Zone. Half-way along this region the cells began to elongate and This was a very long zone which occupied the remaining the nucleus as well as the mitochondria became located in incisal length of the enamel organ of the tooth. Along the the base. An apical cell web with obvious terminal bars length of this zone the amount of dentin increased became evident. At the incisal limit of the region the continually until the pulp became partially occluded. The ameloblasts were fully functional in secreting the initial odontoblasts continued to form a stratified layer and began layer of enamel. The oval nuclei of these cells remained to show signs of regression. The thickness of enamel perpendicular to the layer of mantle dentin and were seen to remained constant up to the point where it was lost by lie at two staggered levels.Mitotic figures in this group of decalcification. The enamel matrix at the conclusion of cells were seen one-third of the way into the region; after outer enamel secretion was homogeneous in staining. this they no longer showed division.. The thickness of the Gradually, rod profiles began to appear and then became stellate reticulum was reduced to one cell layer at the distinct in the region of nearly mature enamel [4] and incisal limit of the region. In the pulp, the previously finally faded rapidly in staining intensity and disappeared differentiated odontoblasts became angled with respect to completely as it became mature enamel. the ameloblast-dentinal junction such that the cell sloped (a) Region of postsecretory transition. toward the apex. Thin wall capillaries moved closer to the For a short distance incisally, immediately after outer odontoblasts as the subodontoblastic layer was lost. The enamel secretion, the ameloblasts remained tall columnar, predentin appeared as a coarse matrix containing bundles but changed their inclination and came to lie perpendicular of fibrils resembling von Korff's fibres and gradually to the surface of the enamel. The mitochondria remained in thickened in amount and became more homogeneous near the infranuclear zone. At this stage, the stratum the incisal limit of the region. intermedium gradually began to lose its organization as a distinct layer. Beyond this short region, the height of the (2) Secretory Zone ameloblast decreased rapidly until the cell was about half (a) Region of inner enamel secretion. its former height. Along with this shrinkage, the base of the The point of initial enamel secretion was marked by the ameloblast and the cells of the papillary layer contained presence of large spherical dark staining bodies between intracellular dense debris[5]. In addition, cells resembling the apices of adjacent ameloblasts. Gradually, the macrophages were occasionally seen in the papillary interdigitating portions of Tomes, processes were layer[6]. delineated by the prongs of enamel matrix and took on the (b) Region of maturation proper. characteristics of inner enamel secretion [3].The After the region of postsecretory transition, the ameloblasts arrangement is described as the "picket-fence" and consists were modified into cells of early maturation in which of processes cut at various angles as they course into the changes were evident at the base and apex of the cell. enamel. The nucleus of the ameloblast appeared to be Foamy vacuolization appeared at the apex of the cell and slightly inclined incisally relative to a perpendicular line the mitochondria began to migrate towards this region. The drawn to the dentino-enamel junction. In the stratum nucleus remained basally located, but the base of the cell intermedium the bilayered and irregular organization of the became irregular. This was caused by the exaggerated basal cells disappeared, and now contained a single continuous bulge and by the large intercellular spaces between the layer of cuboidal cells with large spherical nuclei. The bases of adjacent ameloblasts. The papillary layer increased stellate reticulum and outer dental epithelium were now in height and the papillary cells showed large cytoplasm considered to collectively make up the developing papillary and large spherical nuclei. These cells were not oriented in layer. Labial to this developing papillary layer, the any particular pattern. elongated and attenuated squamous cells maintained their (i) Portions of ameloblasts with striated border. sheet-like appearance. The start of enamel secretion marked The ameloblasts which occupied the longest apical- the beginning incisal length of the incisor had a prominent apical of a distinct predentin-dentin boundary. striated border with heavy concentrations of (b) Region of outer enamel secretion. mitochondria just basal to it. The large oval nuclei of This region occupied a portion of the tooth from the limit the cells were basally located perpendicular to the of inner enamel secretion incisally to a point where Tomes, enamel surface. The chromatin of the nucleus became processes of outer enamel secretion disappeared. The major coarser and more clumped in the incisal limits of this change in this region was in the shape and orientation of portion. The basal bulge of the ameloblast was the interdigitating portion of Tomes' process. These elongated and appeared to extend processes became thinner and longer and were inclined at a finger-like processes towards adjacent papillary cells. greater angle towards the apical end of the incisor. The Large oval intercellular spaces were found between nuclei of the ameloblasts became markedly angled in adjacent ameloblasts at their base. Concentrations of relation to the dentinoenamel junction so that they were mitochondria were seen in papillary cells lying in inclined incisally at about 45°. This angulation was more relation to the basal projections of the ameloblasts. pronounced on the upper than lower incisor. The papillary [7,8] layer increased in height and regularity. The thickness of (ii) Portions of ameloblasts with unmodified apices. the enamel and dentin layers increased progressively.

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Interposed between ameloblasts with striated borders accumulating lateral and proximal to the nucleus. Near the were two or more portions of ameloblasts with end of the region, the cells could be characterized as unmodified apices. These cells showed no striated moderate or heavily pigmented ameloblasts. border and were separated by large, irregular intercellular spaces which ran the entire length of the Ruffle-ended ameloblasts (RA) lateral cell membrane except at the base and apex, In areas with ruffle-ended ameloblasts the slender cells where the membranes were in apposition. At the base exhibited rather even lateral cell surfaces limiting of the cell a small blunt, cone-shaped bulge extended intercellular spaces of varying size. Lateral cell contacts into the underlying papillary layer. Large intercellular were sparse except in their apical 4-6 pm, where a terminal darkly stained granules were often seen within these bar was located and no intercellular spaces could be cells. discerned. Proximally the lateral interameloblast spaces (c) Region of pigmentation. seemed to communicate freely with those of the papillary More incisally, the ameloblasts lost their striated border layer. The apical cytoplasm had a characteristic foamy and acquired dark granular accumulations, presumably the vacuolization, predominantly toward the cell body; and ferritin-rich pigment, in the supranuclear region longitudinal striation could sometimes be seen. In the approximating the position of the Golgi apparatus. Later in electron microscope this latter specialization appeared as a the pigmentation region, large dark granules also appeared ruffled border. Proximal to the ruffled border a heavy in the apical and basal zones as well as in the center of the concentration of mitochondria was found. Several dark cell. Gradually, the base of the ameloblast retracted its bodies were evenly distributed in the supranuclear region, extensive interdigitation with the papillary layer, and as this while a few light bodies of vacuoles were located mainly happened, the basal intercellular spaces disappeared. The juxtanuclearly. The infranuclear region was short, and papillary layer remained prominent up to the point where formed a basal bulge or process which extended into the the large dark granules were accumulating in the papillary layer. The cytoplasm contained a few dark bodies ameloblasts and it was here that this layer showed the first and a welldefined concentration of mitochondria. signs of atrophy. (d) Region of reduced ameloblasts. Smooth-ended ameloblasts (SA) As the ameloblasts decreased in height, the nuclei In areas with smooth-ended ameloblasts the lateral cell decreased in size, became spherical and more closely surfaces were highly irregular giving the cell borders, and packed. When the cell became cuboidal in shape, the dark sometimes the nucleus, an undulated appearance. This was staining granules were either replaced by or were due to the presence of many cytoplasmic bridges transformed into yellowish brown granules which connecting neighbouring ameloblasts. The intercellular accumulated within the cytoplasm. As the cells flattened, spaces, which were very wide, could be followed right up they gradually tipped in an apical direction so that they to the distal cell membrane towards the enamel, but were came to lie almost parallel to the enamel surface. In the not found at the proximal end, where a terminal bar like papillary layer, large capillaries were the first components structure was seen towards the intercellular spaces of the to disappear during atrophy. Concomitantly, the layer papillary layer. The infranuclearcytoplasm extended shrunk in height. In the region of ameloblast modulation laterally in the region of the terminal bars, thus creating the enamel organ consisted of a layer of columnar apparently band-like structure. The apical end of the cells ameloblasts and a well-developed papillary layer . The was unspecialized, and the mitochondria evenly distributed ameloblasts were 40 to 60 ,um tall and about 5 ,um in in the distal half of the cells. In the central lightly stained diameter. Their distal surfaces faced enamel, which was region a considerable number of light bodies or vacuole lost in the course of demineralization and isolation of the was located. The cytoplasmic content of the infranuclear enamel organ except in the apical one third of the region, region did not differ from that of the corresponding part of where remaining matrix proteins were stained to a varying ruffle-ended ameloblasts. Transition between ruffle-ended extent.The border between the ameloblasts and the and smooth-ended ameloblast was rather long - 75 cum to papillary layer was undulated, with the concavities located 100 um - and was characterized by a decrease in the height opposite capillary loops. Everywhere in the region of the ruffled border until it completely disappeared. macrophages were seen extending into the intercellular spaces between the ameloblasts. In the ameloblasts, the Transition between smooth-ended and ruffle-ended elliptical nucleus with one or two prominent nucleoli was ameloblasts. placed in the proximal half of the cell and occupied one In contrast this transition was very abrupt. A ruffled border third to one fourth of the total cell length. Giant zone was developed within a distance of one or two cells ameloblasts with several nuclei were regularly observed, from smooth-ended ameloblasts and the lateral and especially in the incisal part of the region. In cross sections proximal intercellular spaces communicated again. The of such ameloblasts up to 13 nuclei were counted. Incisally apical cell specialization close to the transition border often from about the middle of the region of ameloblast had a dense appearance, and ameloblasts here were defined modulation pigment granules gradually accumulated in the as smooth- to ruffle-ended ameloblasts (S-RA). More ameloblasts. The granules first appeared supranuclearly in incisally, in the area of ruffle-ended ameloblasts, the ruffled close apposition to the Golgi apparatus. As they increased border zone became open and vacuolized.[9] in number they spread to most of the cytoplasm, even

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FUNCTION Ameloblasts are cells which secrete the enamel proteins enamelin and amelogenin which will later mineralize to form enamel, the hardest substance in the human body.[10] Ameloblasts control ionic and organic compositions of enamel. It is theorized that a circadian clock probably regulates enamel production on a daily cycle by the ameloblasts. [11,12] Ameloblasts adjust their secretory and resorptive activities to maintain favorable conditions for biomineralization.[13]

PATHOPHYSIOLOGY These cells are sensitive to their environment. One common example is illustrated by the neonatal line, a pronounced incremental line of Retzius found in the primary teeth and in the larger cusps of the permanent first molars, showing a disruption in enamel production when the person is born.[14]

REFERENCES 1 Jump up^ Takahashi, Satomi; Kawashima, Nobuyuki; Sakamoto, Kei; Nakata, Akira; Kameda, Takashi; Sugiyama, Toshihiro; Katsube, Ken-Ichi; Suda, Hideaki (2007). "Differentiation of an ameloblast-lineage cell line (ALC) is induced by Sonic hedgehog signaling". Biochemical and Biophysical Research Communications 353 (2): 405– 11.doi:10.1016/j.bbrc.2006.12.053. PMID 17188245. 2 Nanci A, Zalzal S, Kogaya Y (1993) Cytochemical characterization of basement membranes in the enamel organ of the rat incisor. Histochemistry 99:321–331 3 Warshawsky, H. 1968 The fine structure of secretory ameloblasts in rat incisors. Anat. Rec., 161: 211-230. 4 Warshawsky, H., and G. Moore 1967 A technique for the fixation and decalcification of rat incisors for electron microscopy. J. Histochem. Cytochem., 15: 542-549. 5 Symons, N. B. B. 1962 Globular structures associated with the completion of the enamel matrix in the rat. J. Dent. Res., 41: 55-60 6 Jessen, H., and H. Moe 1972 The fine structure of macrophages in the enamel organ, with special reference to the microtubular system. Z. Zellforsch., 126: 466-482 7 Kallenbach, E., Y. Clermont and C. P. Leblond 1965 The cell web in the ameloblast of the rat incisor. Anat. Rec., 153: 55-70 8 Reith, E. J. 1961 The ultrastructure of ameloblasts during matrix formation and the maturation of enamel. J. Biophysic. Biochem. Cytol., 9: 825-840. 9 KAJ JOSEPHSEN AND OLE FEJERSKOV Departments of Anatomy, Electron Microscopy, and Dental Pathology and Operative Dentistrv, Royal Dental College, Vennelyst Boulevard, DK-8000 Aarhus C, Denmark 10 Jump up^ Gallon V, Chen L, Yang X, Moradian-Oldak J. Localization and quantitative co-localization of enamelin with amelogenin. J Struct Biol. 2013 Apr 11 Jump up^ Sehic A, Nirvani M, Risnes S. Incremental lines in mouse molar enamel. Arch Oral Biol. 201 12 Jump up^ Zheng L, Seon YJ, Circadian rhythms regulate amelogenesis. Bone. 2013 13 Jump up^ Ten Cate's Oral Histology, Nanci, Elsevier, 2013, page 147 14 Jump up^ Illustrated Dental Embryology, Histology, and Anatomy, Bath-Balogh and Fehrenbach, Elsevier, 2011, page 151, 2, 423 York, Academic Press, Inc.c Press, Inc.

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